The nod could provide a much-needed boost, particularly in Iowa, for the Arizona senator and one-time presumed GOP front-runner whose bid faltered and who now is looking for a comeback.

"I think it matters when one of the most respected members of the United States Senate, and in the pro-life family values community lends his support to my candidacy. I think it has significant impact," said McCain, who was joined by Brownback at the announcement at a downtown hotel.

Brownback, who will serve as one of the campaign's national chairman, was scheduled to travel with McCain to Des Moines, Sioux City and then to Grand Rapids, Mich., later in the day.

Brownback said McCain is the most fiscally conservative candidate, has the best foreign policy experience, was right on the strategy for Iraq and takes a tough anti-abortion stand. He also argued that McCain would appoint strict constructionist judges to the Supreme Court.

"If you want a guy to change Washington, John McCain's the guy to do it. He has been in Washington, but he is not of Washington," said Brownback, a Kansas senator who dropped out of the race for the Republican nomination last month with little money and less support.

It's uncertain how much weight Brownback's backing will carry. While he is a favorite of religious conservatives, he failed to persuade them to embrace him as the GOP's consensus conservative candidate. He spent months emphasizing his opposition to abortion, gay marriage and other issues important to the party's right, but left the race ranking low in polls.

Still, Brownback's backing could signal to evangelical Christians that they can trust McCain and could help solidify McCain's credentials on social issues. The endorsement could be especially important in Iowa, where McCain trails in polls.

Despite a solidly conservative Senate voting record on social issues, McCain has a rocky history with cultural and religious conservatives who make up a significant part of the Republican base - and have proven to be influential in Iowa's GOP caucuses.

He once likened their leaders to "agents of intolerance," but since has taken steps to heal his relationship with the voting group. Still, some are skeptical that McCain will be a loyal Republican who will champion their issues, partly because while his record is clear cut, he's not a high-profile crusader against abortion rights and gay marriage.

In a counterpoint to Brownback's endorsement, conservative leader Pat Robertson on Wednesday backed former New York Mayor Rudy Giuliani, who supports abortion rights and gay rights

Other candidates in the crowded GOP field had lobbied for Brownback's support over the past few weeks.

Mike Huckabee, the former Arkansas governor and Southern Baptist minister who has made strides in Iowa in recent weeks, was widely considered the other Republican most likely to get Brownback's endorsement. It's unclear whether Fred Thompson, the former Tennessee senator trying to emerge as the conservatives' choice, ever got a look from Brownback.

Brownback did talk to Rudy Giuliani, a backer of abortion rights and gay rights, and emerged from the meeting with kind words about the former New York mayor. Brownback spent months this summer bitterly criticizing Mitt Romney's shifts on social issues.

Brownback said McCain was the most electable among the field of Republicans.

"I know the full set of candidates in the Republican Party primary, it is a good field ... but there's one that stands out, that is the full package, that can beat Hillary Clinton in the fall," he said.

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